Electric power generation and distribution systems as employed in the aerospace field typically provide a centralized mechanism to effectively distribute electric power generated from multiple power sources to multiple electrical loads on an aircraft. The power sources may include primary, auxiliary and emergency generators driven by propulsion engines or turbines. The type of electrical loads requiring power for a given aircraft can vary depending on a military or commercial application. Generally, most modern aircraft have numerous flight critical loads such as avionic equipment required for communication and navigation, electromechanical actuation equipment required for manipulation of flight control surfaces, and electric motor driven fuel pumps and control valves. In addition, power may be required for environmental control(s) and de-icing and lighting equipment. All of these can contribute to safety and basic functioning of the aircraft. Moreover, in any particular application, other loads may be present, such as the modern galley conveniences of a commercial airliner or the sophisticated weaponry of a military fighter jet.
Within such a complex and variable electric power system environment, it is sometimes desirable to monitor both the configuration and safe operation of the system. This monitoring can, for example, include determining if the output voltage is controlled within a certain range needed by the loads. By measuring or sensing the amount of current flowing at various points in the system, one can determine whether a voltage drop has occurred and thus, whether an adequate power output level is being sustained for proper functioning of the aircraft systems. In addition, by sensing the level of current in the system at both an input and an output, protection against overloading the entire power and distribution system can be achieved. Without this protection against an overload condition, a fault may develop in one of the various power units. Differential current protection can also be undertaken to determine if a short circuit condition has arisen.
As the electric power levels and complexity of the distribution systems for aircraft increases, a need exists for the capability to measure current at increased power levels. In addition, the ability to sense current magnitude(s) over a broad band of frequencies is often needed, such as where variable frequency power generation and utilization devices are employed.
One method which has previously been used to measure AC current involves the use of a sense winding having an iron core. However, in the case where high current magnitudes are to be sensed, a sense winding with a large number of turns is needed along with a large iron core to avoid premature saturation. As the number of turns is increased along with the core size, an extremely bulky and heavy assembly is created. This significantly adds to the cost of the system, and may even be unworkable for an aircraft depending on spatial and weight constraints.
A device for sensing current in the electric utility industry is been disclosed by Wolf et al. U.S. Pat. No. 4,182,982. In order to measure utility power line current to monitor consumer usage, a transducer is employed which includes a conductive current divider having a branch path. A compensated transformer arrangement is inductively coupled to the branch path. In addition, a magnetic flux balancing arrangement, which includes an amplifier circuit, is provided to virtually compensate the magnetic flux produced by the transformer and provide an output signal. While the Wolf et al. patent teaches sensing current in a fixed frequency system of 60 Hz, there is no disclosure that the device is suited for sensing current in a DC circuit or other frequencies in an AC circuit, as are typically found in the environment of an aircraft electric power generating system.
One such aircraft generating system involves a variable speed, constant frequency (VSCF) system in which a variable speed prime mover (i.e., the engine of the aircraft) mechanically drives a synchronous generator at a variable speed. Because the generator is driven at a variable speed, the frequency of the output power developed thereby is similarly variable. This variable frequency power is typically converted by a rectifier circuit into DC power. An inverter then inverts the DC power from the rectifier circuit into constant frequency AC output power. The sensor disclosed in the Wolf et al. patent would not be useful to sense DC current levels since DC has no frequency component, and hence no magnetic coupling with the transformer in the branched path can occur to provide a compensated output signal. Moreover, the Wolf et al. device utilizes a conductor having a constant cross section throughout and thus may not necessarily provide the desired performance over the broad frequency band needed to adequately monitor aircraft loads.